1. Field of the Invention
The present invention relates to a stacked type electromechanical energy conversion element and a vibration wave driving apparatus.
2. Description of the Related Art
Conventionally, a vibration wave motor such as one 90 shown in FIG. 6 has been used for example, as an autofocusing motor for camera lenses.
In FIG. 6, the vibration wave motor 90 is comprised of a bolt member 91, a vibrator 92 and a rotating unit 93 into both of which a shaft of the bolt member 91 is inserted, and a nut member 94 screwed onto the bolt member 91 to clamp the rotating unit 93 in cooperation with the vibrator 92.
The vibrator 92 is comprised of a metal elastic body 95, a printed circuit board (PCB) 96 connected to an external power supply (not shown), a stacked piezoelectric element 97, and a metal elastic body 98 that cooperates with the elastic body 95 to hold the PCB 96 and the stacked piezoelectric element 97 therebetween. These component parts are arranged in the order mentioned as viewed from a head 91a of the bolt member 91.
The rotating unit 93 is comprised of a gear 99 rotatably supported on the nut member 94 via a ball bearing, a rotor 100 rotatable in unison with the gear 99 and disposed in contact with the elastic body 100, a spring 101 urging the rotor 100 against the head 91a of the bolt member 91 in a direction away from the gear 99, and a spring support 102. These component parts are arranged in the order mentioned as viewed from the nut member 94.
The spring 101 serves to press the rotor 100 against the elastic body 98.
The stacked piezoelectric element 97 has a stacked structure formed of a plurality of ceramic layers (piezoelectric layers) that are stacked one upon another. These ceramic layers have an electromechanical energy conversion function and are formed at their respective one surface with electrode layers (hereinafter referred to as “the internal electrodes”) of an electrode material. With this stacked structure, application of low voltage may result in a high deformation strain and a large force. Such stacked piezoelectric elements are disclosed in, for example, Japanese Patent Publications Nos. 3,311,034, 3,313,782, and 3,432,321.
FIGS. 7A and 7B are views useful in explaining the stacked piezoelectric element 97 of FIG. 6, in which FIG. 7A is a perspective view of the device 97, and FIG. 7B is an exploded perspective view thereof.
Specifically, FIGS. 7A and 7B show a stacked piezoelectric element disclosed in Japanese Patent Publication No. 3,432,321.
As illustrated in FIG. 7A, the stacked piezoelectric element 97 has a circular or disk shape with a central opening formed therein. The dimensions are an outer diameter of 10 mm, an inner diameter of 2.8 mm and a thickness of approximately 2.3 mm. As shown in FIG. 7B, the stacked piezoelectric element 97 is comprised of a first or uppermost piezoelectric layer 110, and second to twenty-fifth layers in which piezoelectric layers 111 and 112 are alternately disposed. The piezoelectric layers 110, 111, and 112 each have a thickness of approximately 90 micrometers (μm).
Each piezoelectric layer 111 has a surface thereof facing toward the first or uppermost layer, which is formed with four segmented internal electrodes A+, B+, A−, and B−. Similarly, each piezoelectric layer 112 has a surface thereof facing the uppermost layer, which is formed with four segmented internal electrodes AG+, BG+, AG−, and BG−. The internal electrodes A+, B+, A−, B− of the piezoelectric layer 111 have the same inner and outer diameters as the internal electrodes AG+, BG+, AG−, BG− of the piezoelectric layer 112. The thickness of each internal electrode is approximately 2 to 3 μm.
The piezoelectric layers 111 and 112 are each formed therein with eight through electrodes 113 with an electrode material filled therein for causing corresponding internal electrodes to electrically conduct. Among these through electrodes, four through electrodes 113 are arranged to cause the respective corresponding segmented internal electrodes A+, B+, A−, and B− of the alternate piezoelectric layers 111 to electrically conduct independently. The other four through electrodes 113 are arranged to cause the respective corresponding segmented internal electrodes AG+, BG+, AG−, and BG− of the alternate piezoelectric layers 112 to electrically conduct independently. The twenty-fifth piezoelectric layer 112, which is the lowermost layer, has no through electrodes.
The through electrodes 113 extend through the stacked structure and have their respective one ends 113a exposed to an outer/upper surface of the first piezoelectric layer 110 where the one ends 113a are in direct contact with the PCB 96 and electrically connected to wiring conductors (not shown) on the PCB 96.
The stacked piezoelectric element 97 performs polarization by applying positive voltage to the internal electrodes A+ and B+ out of the four segmented internal electrodes of the piezoelectric layers 111 and negative voltage to the internal electrodes A− and B− of the same with the internal electrodes AG+, BG−, AG− and BG− grounded such that the paired internal electrodes A+, A− and the paired internal electrodes B+, B−, each pair being offset by 180 degrees, are opposite in polarity, i.e. one is positive, and the other is negative. The vibration wave motor 90 applies high-frequency voltage almost equal to the natural frequency of the vibrator 97 to an A phase to which the electrodes A+ and A− correspond, with the electrodes AG+, AG− corresponding to an AG phase and facing the A phase and the electrodes BG+, BG− corresponding to a BG phase and facing a B phase to which the electrodes B+ and B− correspond, the B phase being different by 90 degrees in spatial phase from the A phase, being grounded. Further, the vibration wave motor 90 applies high-frequency voltage equal in natural frequency to and electrically different in phase by 90 degrees from the high-frequency voltage applied to the A phase to the B phase. This causes the vibrator 92 to generate two bending vibrations intersecting with each other, whereby driving vibrations are obtained by synthesis of the generated two bending vibrations to thereby frictionally drive the rotor 100 disposed in urging contact with one end face of the elastic body 98 via the spring 101.
As stated above, the stacked piezoelectric element 97 of the conventional vibration wave motor 90 has the internal electrodes of uniform dimensions (inner and outer diameters) throughout the stacked structure. This is because the stacked piezoelectric element 97 will produce a relatively even strain distribution though there are some variations (a1), (b1) and (c1) in strain depending on locations in the device 97, as shown in FIG. 8, and hence the internal electrodes can be uniform in dimensions. FIG. 8 is a fragmentary vertical sectional view of a half of the piezoelectric element in the radial direction, useful in explaining strains over the piezoelectric element.
Recently, there is a strong demand for a vibration wave motor of this type to be more compact in size, higher in efficiency, and higher in output so as to be applicable to various appliances, as disclosed in Japanese Laid-Open Patent Publications (Kokai) Nos. 2003-134858, 2003-199376, and 2003-209983.
Vibration wave motors disclosed in the above-mentioned publications are still insufficient in efficiency and output, leaving room for improvement of the stacked piezoelectric element thereof. Specifically, analysis of the structure of the stacked piezoelectric element has revealed that in small-sized vibration wave motors, to produce efficient vibrations of the vibrator, strains generated in the stacked piezoelectric element should have such a distribution that the strains largely vary in the thickness or stacked direction and in the radial direction of the stacked piezoelectric element.
That is, if all the internal electrodes of the stacked piezoelectric element are configured to have uniform dimensions as in the conventional stacked piezoelectric element, strains occur even in a region where inherently strains should not be generated or a region where inverse strains, i.e. strains caused by compression should be generated, which can cause loss of supplied electric power, and hence degraded operating efficiency and insufficient output of the vibration wave motor.